The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, an automatic transmission achieving more speed stages has been developed to enhance fuel efficiency and improve drivability. Recently, increases of oil price has triggered more competition for enhancing fuel efficiency of a vehicle.
In this regards, many researches for reducing weight and enhancing fuel economy through downsizing of an engine and for securing drivability and fuel economy through multiple speed stages of automatic transmissions have been conducted.
However, in the automatic transmission, as the number of speed stages increase, the number of internal components (particularly, planetary gear sets) increase, and as a result, a length of the transmission increases. Therefore, mountability, cost, weight, transmission efficiency, and the like may still deteriorate.
Accordingly, development of a planetary gear train which may achieve improved efficiency with reduced number of components has been considered in order to increase a fuel efficiency through the multiple-speeds.
In this aspect, in recent years, 8-speed automatic transmissions tend to be implemented for vehicles, and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted.
However, a conventional 8-speed automatic transmission typically includes three to four planetary gear sets and five to seven engagement elements (frictional elements). In this case, since the length of the automatic transmission increases, mountability may be deteriorated.
We have discovered that disposing one planetary gear set above another planetary gear set causes significant limitations to structures of automatic transmissions to which parallel planetary gear sets is applied.